Study On The NO_x Removal Performance And Mechanism Over Fe-ZSM-5 Catalysts In The SCR Reaction

NO_x emission of diesel engines threatens human health and has detrimental effects on the ecological environment.Selective catalyst reduction（SCR）technology is known as one of the most effective methods to remove NO_x emission from the diesels,and the Fe-based zeolite SCR catalyst has been extensively studied for its excellent reaction activity under high temperature and high activity for N₂O catalytic degradation.To meet the requirements of increasingly strict emission regulations,developing more advanced SCR catalysts with wide active temperature window and better hydrothermal stability is important for the development of diesel vehicles.NO_xremoval mechanism over various Fe sites for the Fe-based catalysts lays a foundation for the design and development of SCR catalysts.And,the active Fe and acid sites are important media for SCR reaction.Therefore,this paper studied the factors affecting the catalytic activity of Fe-based SCR catalyst and the NO_x removal mechanism based on the active sites.The mian research work and results are as the following:Firstly,Fe-ZSM-5 SCR catalysts with different Fe contents were prepared by impregnation method.SCR catalytic activity was evaluated based on the fixed bed reactor.Various Characterization methods,including X-ray diffraction（XRD）,NH₃ programmed temperature desorption（NH₃-TPD）and Ultraviolet visible spectrum（UV-Vis）etc.were used to investigate the role of structure,active Fe and acid sites in SCR reaction.The possible intermediate species and reaction pathway in SCR reaction were further analyzed by turn over Frequency（TOF）calculation,in-situ diffuse infrared Fourier transform spectroscopy（DRIFTS）and density functional theory（DFT）.The results show that the NO_x conversion of Fe-ZSM-5 decreased first and then increased with the increase of Fe content at low temperature below 500℃,decreased with the increase of Fe content at high temperature,and the catalyst presented the best catalytic activity when the Fe content was 1.0%.The results of catalytic characterization experiments,catalyst performance evaluation and TOF calculation show that monomeric Fe species(isolated Fe³⁺)were the main SCR active sites of Fe-ZSM-5 at medium and low temperature,while the dimeric Fe species and Fe_xO_y were acive for NH₃ oxidation under high temperatures.The NH₃-SCR activity was not only influenced by the amount of active Fe species,but also dependent on the relative content of active Fe species.At the same time,NH₃ adsorption and desorption,which depends on the acid sites,also played an important role in NO_x removal.The“L-H”and“E-R”mechanism could be both applied to the SCR reaction for Fe-ZSM-5 catalysts.In addition,for NO₂+NH₃ reaction,the reaction pathway of NO₂ and NH₃ at Fe sites was mainly realized by the formation of three kinds of complex species with different structures.Then,Fe-ZSM-5 catalysts calcined at different temperatures were prepared.The NO_x removal mechanism over the catalysts was studied by analyzing the NO_x removal performance and the results of XRD,NH₃-TPD,UV-Vis and other characterizations over the catalysts.The results show that the Fe species in the Fe-ZSM-5 catalysts agglomerate during the high-temperature heat treatment,which changed the physiochemical properties and catalytic activity of the catalyst.The NO_x removal performance of 1.0Fe-ZSM5 catalyst increased from 500℃to 600℃,while when the calcination temperature reaches 700℃,the NO_x conversion at high temperature began to decrease.The main active sites of SCR reaction at low temperature below 500℃were isolated Fe³⁺,which was more stable during calcination and hydrothermal aging than[HO-Fe³⁺-O-Fe³⁺-OH]²⁺.In the temperature range of 300-500℃,dimeric Fe species participated in the SCR reaction as one main active site.However,[HO-Fe³⁺-O-Fe³⁺-OH]²⁺had certain oxidation activity,and the isolated monomer Fe³⁺was more active than the dimers.At high temperature above 500℃,the main active sites were[HO-Fe³⁺-O-Fe³⁺-OH]²⁺,and the dimeric[HO-Fe³⁺-O-Fe³⁺-OH]²⁺together with Fe_xO_y particles could be used as the active sites for NH₃ oxidation.With the increase of calcination temperature,Lewis acid sites moved to lower temperature,which improved the NO_xremoval performance at low temperature.Finally,the catalytic activity of Fe-ZSM-5 catalysts was evaluated after thermal aging and hydrothermal aging.The influence of H₂O and heat on SCR activity was studied by XRD,NH₃-TPD,UV-Vis and NMR MAS.Compared with fresh samples,SCR catalytic performance of 1.0Fe-ZSM-5 aged at 750℃has been improved obviously,while the NO_x removal performance of the catalyst hydrothermal aged under 750℃with 10%H₂O was reduced within a wide temperature range.The results show that the SCR activity of Fe-ZSM-5 catalyst could be improved by moderate thermal aging,while the presence of H₂O in hysrothermal aging is harmful to the SCR activity of the Fe-based catalyst.During hydrothermal aging,Fe species gathered and integrated into large clusters and particles of Fe oxides,and the high temperature and H₂O promoted the aggregation and migration of Fe species.During the thermal aging at 750℃for 16 h,the free Fe²⁺was transformed into[HO-Fe³⁺-O-Fe³⁺-OH]²⁺,and further decomposed into isolated Fe³⁺,which promoted the SCR activity of the catalyst.At the same time,the dealumination of catalyst caused the loss of Br（?）nsted acid site and the decrease of specific surface area of activity.Br（?）nsted Acid sites are important for SCR reaction at high temperature.The intensity of Br（?）nsted Acid was destroyed after hydrothermal aging,which resulted in the significant reduction of NO_x conversion at high temperature.